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<br />the 386 computer, run times for a three hour flood simulation with <br />500 grids, 150 ft on a side, was on the order of 10 to 30 minutes. <br />For larger grid systems or smaller elements, run times of several <br />hours may be expected. Data preparation for a flood simulation <br />with 500 grid elements was less than a week. <br /> <br />5.3 Model Theory <br /> <br />FLO-2D predicts flow depth <br />continuity equation Eqn. 1 and <br />equations Eqns. 2 and 3: <br /> <br />and velocity by solving the <br />the two-dimensional momentum <br /> <br /> ah ahVx ahVy <br /> + + = i <br /> at ax ay <br /> ah Vx avx vy avx 1. avx <br />Sfx = Sox - ,- <br /> ax g ax g ay g a1: <br /> ah vy avy Vx avy 1. avy <br />Sfy = SOY - ,- <br /> ay g 8y g ax g 81: <br /> <br />(1) <br /> <br />(2) <br /> <br />(3) <br /> <br />where 1'1 is the flow depth and Vx and vy are the depth-averaged <br />velocity components along the coordinates, x and y. The excess <br />rainfall intensity i can be nonzerofc)r both the watershed and fan. <br />The friction slope components Sfx and SXY depend on the bed slope Sox <br />and Soy and the last four terms in Eqns. 2 and 3 which describe the <br />pressure gradient, two convective and one local acceleration term, <br />respectively. <br /> <br />The solution of these first three equations constitute the <br />basis for computing flow hydraulics and routing a water flood over <br />a grid system. The diffusive wave approximation to the momentum <br />equation is obtained by neglectin~J the last three acceleration <br />terms of Equations 2 and 3. The diffusive pressure gradient in the <br />momentum equation is an important: parameter in improving the <br />accuracy of simulating flow over diverse topographical condi.tions. <br />This approximation is valid for steep alluvial fans and their <br />contributing watersheds. An option tlJ (~mploy the full dynamic wave <br />equation is being tested. <br /> <br />To route mud floods and mudflOYls, the rheological behavi.or of <br />the flows must be defined. The fluid properties of mudflows <br />rapidly vary, making accurate prediction of the properties a <br />difficult task. Mudflows are charac.terized by surging and abrupt <br />flow stoppage. Due to the transient nature of t.hese phenomena, the <br />most practical approach is to tn!at the flow as a continuum, <br />combining the water and sediment components. <br /> <br />The rheological behavior of hyp,arconcentrated sediment flows <br />(mudflows) includes the interaction of several complex fluid <br />processes. The nonNewtonian behavior of the fluid matrix is <br /> <br />19 <br />